Did you download it from the aeria client while steam was up? My suggestion is to check in (drive):/aeriagames/ava to see if the steam_api.dll is there. If it is not, you have two options. You can either uninstall ava and disable steam client for about a few hours or a day, then download ava through the aeria client or you can download steam_api.dll file and use regsvr32 to register the dll.
If the steam_api.dll file is there, you can use the regsvr32 commands to register it and should resolve the issue.
regserver prompt codes and examples in the link below if you decide to do it manually.